This application claims the priority of Korean Patent Application No. 10-2004-0090126, filed on Nov. 6, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an apparatus for measuring body fat by using biomedical impedance, and more particularly, to a method of and an apparatus for measuring body fat, by which measurement errors can be reduced and repeatability can be improved by compensating for variation of a contact voltage caused by variation of an electrode contact area.
2. Description of Related Art
A body fat value serves as a main index to determine fatness, which is known to cause many adult diseases. As interest in fatness is increasing and the necessity of measuring body fat is increasing, a body fat measurement apparatus is becoming pervasive.
Today, a method of measuring bioelectrical impedance is most frequently used to measure body fat. This method is based on the fact that bioelectrical impedance varies depending on the amount of body fat.
Typically, since fat occupies 17% of a human body and contains little water, the bioelectrical impedance of a human body varies depending on the amount of body fat. Therefore, it is possible to measure the amount of body fat by detecting the bioelectrical impedance of a human body.
A typical body fat measurement apparatus is a four-electrode type having two current electrodes and two voltage electrodes. The four electrodes usually make contact with parts of a human body, for example, the hands. In the typical body fat measurement apparatus, the two current electrodes are used to apply a constant current through a human body, and the two voltage electrodes are used to measure a voltage drop caused by the constant current. The voltage drop across the voltage electrodes is proportional to the bioelectrical impedance of a human body.
FIG. 1 schematically illustrates an example of a portable body fat measurement apparatus. The portable body fat measurement apparatus has a display unit mounted on a front surface of a chassis, an operation unit, and measurement electrodes mounted on both sides of the chassis. As shown in FIG. 1, the four electrodes are mounted on the left and right sides, two by two, respectively. For example, the voltage electrodes are mounted on the front left side and the front right side, and the current electrodes are mounted on the rear right side and the rear left side.
FIG. 2 schematically illustrates a principle of measuring body fat by using a four-electrode type body fat measurement apparatus. Two current electrodes 102a, 102b are connected to a constant current source 104, and two voltage electrodes 106a, 106b are connected to a voltage meter 108. When fingers of the left and right hands make contact with the current electrodes 102a, 102b, respectively, the constant current applied from the constant current source 104 flows through the right hand, the upper-half of a human body, and the left hand. As a result, a voltage drop V occurs due to the bioelectrical impedance of a human body. The voltage drop V is proportional to a body fat ratio. Then, the voltage drop V caused by the impedance of a human body is measured by using the voltage meter 108 through the voltage electrodes 106a, 106b making contact with the fingers of the left and right hands of a user.
Subsequently, a user inputs personal information such as the height, weight, age, and sex of a user, into the body fat measurement apparatus, and the voltage measured by the voltage meter 108 is input to a predetermined calculation function to calculate the amount of body fat. Finally, the measurement result is displayed on the display unit.
Therefore, it is considered that the measurement accuracy depends on the user's posture, contact areas with the measurement electrodes, and the like. Particularly, for a portable body fat measurement apparatus, it is recommended that the fingers be spread out as much as possible and the user's posture be stabilized as shown in FIG. 1.
In the meantime, the contact area means the area over which a user makes contact with the measurement electrodes, i.e., the voltage electrodes 106a, 106b. Experimental observation has shown that variation of the contact area significantly affects the measurement result. Such variation of the contact area alters the contact voltage between a human body and the voltage electrodes, and thus the variation of the contact voltage alters the voltage measured from the voltage electrodes 106a, 106b. It is considered that the contact area of the measurement electrode is proportional to the finger contact pressure applied when the measurement electrode is pressed.
FIG. 3 schematically illustrates variation of a body fat value depending on the contact area. The left of FIG. 3 shows a situation where the contact area is sufficiently large, and the right of FIG. 3 shows a situation where the contact is smaller than that. Referring to the left of FIG. 3, if the contact area, i.e., the contact pressure, of a user increases, the contact voltage correspondingly increases, so that the measurement body fat value is measured to be larger. On the other hand, if the contact area, i.e., the contact pressure, of a user decreases, the contact voltage correspondingly decreases, so that the measurement body fat value is measured to be smaller.
Also, it has been observed that such a measurement deviation caused by the variation of the contact area reaches about ±2% of the true body fat value. This means that a person having a weight of 64 Kg would have a deviation of ±1.2 Kg.
Since the contact area may change for every measurement, it is necessary to compensate for such variation of the contact area. Otherwise, measurement consistency of the body fat measurement apparatus cannot be ensured. In other words, if the variation of the contact area occurs, repeatability cannot be guaranteed even when conditions other than the contact area are sufficiently satisfied.